ATF Question

A few months ago I got this bright idea to buy an inexpensive shooter and convert it to an artillary model. One of the suppliers had some long barrels available and I knew Numrich had the stocks. A lot of you guys sent me advice on which model was best to convert, how to make the rear site etc. John S. mentioned that the ATF might frown on it and that it might be a violation of the law. That advice alone was enough to scare me away from the project. We were just talking about it the other night and one of my shooting buddies asked why it would be against the law. I couldn't answer him. I'm probably missing something here but can anybody give me a reason why putting a long barrel and a stock on a pistol would be risky in the eyes of the ATF.